Understanding VFD & 3 Phase AC Motors: Torque, Speed, and Voltage Explained

In summary, the conversation discusses the concept of VFD and AC motors running on three phase. The torque is directly proportional to the applied voltage squared and the speed is directly proportional to the frequency. One way to control motor operation is by using the rule of volts per hertz, where the applied voltage is adjusted proportionally with the frequency to maintain the same current from the supply. The conversation also raises questions about the meaning of "motor on load" and how to calculate the starting torque required for a motor. The conversation suggests using the Steinmetz equivalent circuit to answer these questions.
  • #1
dannygul
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Hi,

I am trying to understand the concept of the VFD & the AC motors running on three phase . I have briefed about the concept involved with the VFD what i understood;---

Torque --- directly proportionalt to applied V^2
speed --- directly proportionalt to frequency

One way to control the industion motor operation is using following rule :---
volts per hertz
applied voltage to be proportionally adjusted whenever the frequency is changed to draw same current from the supply.

means torque & speed can be adjusted by varying the voltage & frequency.


I am confused at two thing :-----

1> what do we means by the -- motor on load ?


2> how can we find the starting torque required by the motor, when suppose some device is connected to it ?

For 2> i have done following analysis but still stuck at one thing.
Suppose for example i want to rotate the hollow cylindrical attached to shaft of my motor. Then first i need to find out the -- Torque needed to start the rotation of hollow cylinder.
Litle physics :----
Torque = dL/dt = d(I*w)/dt = I*d(w)/dt = I* α

moment of inertia of a uniform hollow cylinder of inner radius b, outer radius a, and mass M.
I = 1/2* M * (a^2 + b^2 )

But how can we calculate the α angular acceleration well in advance when motor is not started ?


Please suggest for these point.
 
Engineering news on Phys.org

1. What is a VFD and how does it work?

A VFD, or variable frequency drive, is an electronic device used to control the speed of an AC motor by adjusting the frequency of the electrical power supplied to it. It works by converting the incoming AC power to DC, and then using pulse width modulation (PWM) to create a variable frequency AC output.

2. What are the benefits of using a VFD with a 3 phase AC motor?

There are several benefits to using a VFD with a 3 phase AC motor. These include improved energy efficiency, precise speed control, and the ability to start and stop the motor smoothly. VFDs also help to reduce wear and tear on the motor and other mechanical components.

3. Can a VFD be used with any type of 3 phase AC motor?

Yes, a VFD can be used with most types of 3 phase AC motors, including induction motors, permanent magnet motors, and synchronous motors. However, it is important to select a VFD that is compatible with the motor's voltage and current ratings.

4. How do I determine the correct size of VFD for my 3 phase AC motor?

The size of the VFD needed for a 3 phase AC motor depends on several factors, including the motor's power rating, speed range, and load characteristics. It is recommended to consult with a qualified engineer or VFD manufacturer to determine the appropriate size for your specific motor and application.

5. Are there any safety considerations when using a VFD with a 3 phase AC motor?

Like any electrical equipment, there are some safety considerations when using a VFD with a 3 phase AC motor. It is important to follow proper installation and wiring procedures, and to ensure that the VFD is properly grounded. Additionally, users should be aware of potential hazards when working with high voltage and follow all safety precautions.

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